Linear modulator



Nov. 23, 1943. J. A. RANKIN LINEAR MDULATOR 2 Sheets-Sheet 2 Filed ned.

. tion potential Patented Nova-23, 1943 d LINEAR MoDUL'roit John A. Rankin, Port washington, N. Y., signor to Radio Corporation of America, a corporation y of Delaware Application December 5, 1941, Serial N o. 421,711

9 claims. (c1. 17a- 1115) This application concerns a new and improved methodof and means for generating wave energy and modulating the wave length thereof substantially linearly in accordance with modulating potentials.

More in detail, my disclosure concerns wave length modulators of the type wherein a reactance tube controls the frequency Aof operation of a generator of wave energy, the reactance tubev mutual conductance being in turn controlled by` modulating potenials. A feature of my invention is the use ofan impedance in -the reactance tube cathode circuit wherein a potential is produced which varies in accordance with variations in the current owing in the reactance tube. The

saidvariations thus are an indication of modulation potential and/or reactance tube distortion and are supplied to an early stage in the modulaamplifler in a degenerative sense to correct the said distortion so that the wave length modulation of the oscillations is truly representative of the modulating potentials.

The invention is of wideA application in the radio ar-t and may be used in frequency modulators of the Crosby type (Crosby Patent #2,279,659, dated April 14, 1942) wherein the mean frequency lof operation of the wave length modulated wave gener tor is stabilized by deriving from the modulated scillations a potential which varies in accordance with the variations of the mean freguency thereof, from the correct mean frequency,

and applying said derived potentials as a control potential on the reactance tube. By the use of such stabilization in combination with my improved distortion correction method and lneans,

improve/d results are obtained in that substan tially linear wave length modulation of the oscillations generated is obtained.-

In describing my invention in detail, reference `be made to the attached drawings wherein: FigQl illustrates a reactance tube modulated generator wherein a potential is derived in the cathode circuit of the reactance Atube and applied degeneratively to an earlier modulation potential stage; y

Fig. 2 is a. 'modification of Fig. land includes means for stabilizing the mean frequency of the generated wave; while Fig. 3 is a modied form of feedback-circuit' which may be used to replace the feedback circuits of Figs. 1 and 2.

In Fig. 1, 2 is a reactance tube, I0 is a wave generator the frequency of operation of which is to be controlled by the reactance tube 2, 20 is' the first stage of a modulation potential am- `50 tion necessary to 55" the voltage on the anode A. Thus plier, the second stage 30 of which is coupled to an electrode of the reactance tube, and RC is a resistance-condenser combination wherein a potetial is produced and supplied by line 40 to the 5 control grid of tube 20;

The oscillation generator may be of any approved type and may, for example, comprise a tube Il! having its anode I l and grid l2 connected in a regenerative oscillation circuit including tun- `ing condenser C4 and tank inductance LI. The

operation of the generator is controlled by the reactance tube 2 which again may be of any approved type such as, for example, of the nature disclosed in Crosby United States vapplication No.

l5 136,578, filed April 13, 1937, now U.`S. Patent No.

2,279,659, dated April 14,

This reactance tube has to a 'point on circuit Ll--CI and its cathode connected to a second point on said circuit by relsistance R and radio frequency bypass condenser C so that the impedance of this tube, that is, the space between its anode and cathode is connected across the frequency determining circuit of the oscillation generator. The anode A is connected by Va coupling condenser CC and phase shifting resistance RO and phase shifting condenser CO to ground. A point between RO and CO is connected to the grid GI. The purpose of this phase shifting circuit is to provide a substantial phase quadrature relation between the high frequency voltage on anode A and the current to the anode within the tube, and this circuit comprises RO, the capacity between the grid GI and cathode K takenwith condenser CO and also the capacity of circuit connections, etc.

Thus resistor RO and condenser CO form a phase shifting circuit whichA shifts the voltage supplied from the tank circuit LI-C4 to the plate A of the reactance'tube 2 so that it reaches the grid GI with a substantially 90 degree phase lag. The value of ,RO is made high as compared to the reactance of CO for the frequency generated xso that current through the circuit RO-CO is j .larger resistively and is in phase with the voltage. H owever,

the voltage drop through CO lags the current by substantially degrees andthe current to the plate A lags the' plate voltage by about 90 degrees. Thus the desired phase rela- This reactive effect is complex but with the particular connections shown is inductive since' the A is lagging current within the tube to the anode the tube 2 its anode A connected l produce the reactive eect rather than a resistive effect in -tube 2 is obtained.,

provides a reactance which is included in the frequency-determining circuit of the generator. q The size of the reactance provided by the tube 2 .is controlled by controlling the conductivity of the tube 2 and this is controlled by modulating potentials from transformer T| impressed on the grid 2| of tube 20 wherein they are amplified and supplied by coupling condenser CC to the grid 29 of tube 30 and thence by transformer T2 to the grid GI. The amplifiers 20 and 30 are usual and include in the cathode circuits thereof biasing units 23 and 3| and also include other usual features such as bypass condensers BP and the necessary charging resistances, etc.

As stated hereinbefore, the cathode of the reactancetube 2 is returned to ground through a network RC, C being a complete bypass at the frequency of operation of the generator but bee ing small at the modulation frequency so that voltages of modulation frequency and below are developed across R. These voltages are fed back by the lead 40 and resistor I9 to the control grid 2| of the stage 20 of a phase determined by the transformer T2. The phase of the feedback voltages is made such by properly poling the windings `of transformer T2 that it opposes the modulation voltage to provide degeneration in the amplifier, reducing the gain thereof and the amount of distortion in the system.

The potential provided by RAC and supplied to the initial stage by the lead 40 reduces the distortion inherent in the audio amplifier, made up of tubes 20 and 30, 'and also reduces the distortion inherent in the reactance tube 2. The circuit further improves the frequency response characteristic of the audio amplifier, thereby giving improved performance to the modulator unit. Note in my system that by taking voltage from the plate cathode circuit of the reactance tube 2, compensation of the non-linearity inherent in the reactance tube is assured. l

The modification ofv Fig. 2 is in many respects similar to the modification of Fig. 1. In the two figures, corresponding elements have been designated by corresponding reference numerals and lette`rs and a detailed description of Fig. 2 is believed unnecessary.

, However, in Fig. 2 it win be noted that the" phase shifting circuit of the reactance tube 2 now comprises a condenser CO between the anode and grid Gl and a resistance RO between the grid Gl and` ground. In this arrangement the'- resistance RO is fsmall as compared to the reactance of CO and thevcurrent through the condenser CO leads the voltage thereacross so that the voltage on the grid GI leads the-voltage on the anode A by aboutA 90 degrees and the current to the anode A leads the voltage thereon, and the desired reactiver effect' in tube 2 is obtained.l

Since the current to the anode leads the voltage thereon, the reactive effect is capacitive in nature although it will be understood that these tubes provide complex reactances.

The oscillator l in Fig. 1 is of the 'grounded `grid type whereas in Fig. 2 it is of the grounded anode type with the cathode 1 raised above ground potential by radio frequency choke RFC. Moreover, in Fig. A2' the output of the oscillator power ampliers and/or is shown as comprising amplitude hunters and/or frequency multipliers.

as required, all being included in the unit I3.

The generator I0 in Fig. 2 has its mean frequency of operation stabilized in a manner slmilar to that used by Crosby in his United States.

application No. 136,578 Anow U. S. Patent No.

picks up modulated oscillations from the osclllator A or from a second stage included in I3 and supplies itl to a wave deviation detecting means in unit 43. This means may comprise, if del sired, wave frequency converting means anda local source of oscillations to mix with the frequency modulated oscillations picked up at 4| and supplied to 43. Alternatively, this means may include a modulated wave amplifier and/or frequency divider. I In any case, the modulated oscillations of a lower frequency or of the frequency pickedup at 4| is supplied toa frequency counting circuit of some type'such as a frequency discriminator of the two-path type or olf-tuned circuit type wherein the frequency variations are converted to amplitude variations and detected in a pair of detectors such as, for example, shown at 50 and 60, so that a differential potential is set up across the resistances 5| and 55, shunted by condensers 53 and 51. The difference of the rectified current is supplied by resistance RI and CI to the grid GI by way of the secondary winding of transformer T2. The time constant circuit Rl-CI is made such that it passes only slow variations in potential characteristic of changes in the mean frequency of the modulated oscillations but does not respective to potentials of modulation frequency.

Comparing circuits I and 2, circuit 2 employs in addition to degeneration vfrom the reactance plate cathode circuit to the input of the audio 3 5 amplifier a second circuit which determines accurately'the mean frequency of the main oscillator. Direct current and audio voltage derived i 50 and 60 of Fig. 2 are fed to the `deriv'ed'so far as linearity' is concernedfrom the vv"application of voltage rectified by tubes 60 and 0 will enhance the improvement derived by the degenerative connection between the plate` cathode circuit .of reactance tube 2 and the input of the audio amplifier comprising tubes 20 and 30.

The arrangements of Figs. 1- and 2. maybe modied as shown in Fig. 3, wherein the potential drop across RC is supplied by way of line 40 to the cathode 25 of tube 20 in place of the grid GI. Degeneration may be accomplished in this manner because RC is 'a low impedance source and can be coupled backy directly to the cathode of the tube 2|.

f .What is claimed is:

1. In a signaling system, a tube reactance the -reactive value of which is controllable, a source of control potentials, a tube amplifier, having a control electrode, coupling said source of control potentials tosaid reactance tube, an impedance actance tube, and a circuit degene Nively cou pling said impedance to the control lectrode in said tube amplifier.

reactance which is included separate and independent so that improvement 7d in the path between two electrodes of said re for determining the reactance of the signaling circuit, a source of control potentials, a multiple stage tube amplifier coupling said source of control potentials to said reactance tube, an impedance in the path between two electrodes of said reactance tube, and a circuit degeneratively coupling said impedance to an early stage of said amplifier.

3. In a signaling system, a tube reactance the reactive value of which is controllable, said tube having output electrodes between which said reactive effect appears and havinga control elec.- trode, a source of control potentials, an amplier tube having an input coupled to said source of control potentials and an output coupled to the control electrode of said reactance tube, and a circuit degeneratively coupling the output electrodes of said reactance tube to the input of said amplier.

4.1m a signaling system, a controllable tube reactance which is included in a signaling circuit for determining the reactance ofthe signaling circuit, a source of control potentials, .an ampliner including a plurality of multiple electrode tubes coupled in cascade and coupling said source of control potentials to said reactance tube, anl impedance in a path between two electrodesl of said reactance tube, and a circuit degeneratively coupling said impedance to an electrode of a tube in an early stage of said amplifier.

5. In a signalling system, a tube reactance the reactive value of which is controllable, a source of control potentials, a -tube amplifier having a control electrode, connections including said tube amplier coupling said source of control potentials to said reactance tube, an impedance in the `path between two electrodes or' said reactance tube, and a circuit degeneratively coupling said impedance to the control electrode in vsaid tube ampliiler.

6. In a signalling system, a controllable tube reactance which is included in the frequency dev termining circuit of an oscillation generator, said impedance connected to the cathode of said resaid tube amplier, and connections for controlling the frequency of operation of said generator in accordance with deviations in the mean frequency of operation thereof.

7. In a'signalling system, a source of control potential, a tube reactance the react-ive value of which is controllable,said reactance tube having electrodes including a cathode and an output electrode, an impedance in a circuit connected between said cathode and output electrode, a tube amplifier, having a control grid and a cathode, coupling said source of control potentials'to'said reactance tube, a connection between the cathode of said tube Vaxriplier and the end of said im- Y I ,pedance remote from the cathodeof -said tube reactance and a circuit connecting the end of said actance tube to the control grid of said tube ampliiier.

8. In a signalling system, a tube reactance the reactive value of which is controllable, said tube reactance having an outputvelectrode and cathode, a source of control potentials, a. tube amplicathode, a source fier having a control grid and a cathode, connections coupling 'said source of control potentialsto said tube amplifier, connections coupling the said tube amplifier to said reactance tube, an impedance in a connection betweenthe output electrode and cathode 'of said reactance tube, a connection between the reactance tube cathodeend of said impedance and the cathode of said ampliiler tube and a connection between the control grid of said amplifier tube and the end of said impedance remote from the ,cathode ,of said reactance tube.

A9. In a signalling system in combination a tube reactance the reactive .value of which is controllable, said tube having electrodes including a amplifier coupling said source-of control potentials to said reactancetube, said tube amplifier having a control electrode and cathode, a rereactance tube having a plurality of electrodes. a

source of control potentials, a tube amplifier,

having a control electrode, coupling said sourcef' of control potentials to said reactance tube,lan impedance in a path between two electrodes o! said reactance tube, a circuit Adegenerativeiv coupling said impedance to the control electrode in sistance in the cathode return circuit of` said reactance tube, and connections between said resistance and the control grid and cathode of 'said tube ampliner for utilizingthe voltage 'developed across said resistance to control the gain f of said amplifierin a degenerative sense.

JOHNl A. Raum.

of control potentials, a tube 

